Fix WITH attached to a nested set operation (UNION/INTERSECT/EXCEPT).

Parse analysis neglected to cover the case of a WITH clause attached to an
intermediate-level set operation; it only handled WITH at the top level
or WITH attached to a leaf-level SELECT.  Per report from Adam Mackler.

In HEAD, I rearranged the order of SelectStmt's fields to put withClause
with the other fields that can appear on non-leaf SelectStmts.  In back
branches, leave it alone to avoid a possible ABI break for third-party
code.

Back-patch to 8.4 where WITH support was added.

src/include/nodes/parsenodes.h

@@ -1016,7 +1016,6 @@ typedef struct SelectStmt
 	List	   *groupClause;	/* GROUP BY clauses */
 	Node	   *havingClause;	/* HAVING conditional-expression */
 	List	   *windowClause;	/* WINDOW window_name AS (...), ... */
-	WithClause *withClause;		/* WITH clause */
 
 	/*
 	 * In a "leaf" node representing a VALUES list, the above fields are all
@@ -1036,6 +1035,7 @@ typedef struct SelectStmt
 	Node	   *limitOffset;	/* # of result tuples to skip */
 	Node	   *limitCount;		/* # of result tuples to return */
 	List	   *lockingClause;	/* FOR UPDATE (list of LockingClause's) */
+	WithClause *withClause;		/* WITH clause */
 
 	/*
 	 * These fields are used only in upper-level SelectStmts.